It is known that the graphite epoxy structural components used on aircraft are subjected to natural lightning strike discharges. The most severe strikes are directed toward small-radius structures located at or near extending surfaces of the aircraft (wing tips, stabilizer tips, vertical tips, rudder, elevators, ailerons, engine cowlings, etc.). These locations are designated as "Zone 1" and are subjected to the initial attachment of the lightning channel. The initial attachment lightning stroke is characterized by a fast-rise, high-peak current (2.times.10.sup.5 amps) and a large energy transfer (2.times.10.sup.6 amps.sup.2 sec). It can cause severe structural damage to unprotected graphite epoxy structures (much more damage than to comparable aluminum structures).
Other parts of the structure will be subjected to lesser discharges, called "swept stroke lightning." These areas are designated as "Zone 2" and are located aft of the original attachment points. Swept stroke lightning also contains a fast-rise current, but with a 1.times.10.sup.5 amp peak and an energy transfer of 0.25.times.10.sup.6 amps.sup.2 sec.
Integral fuel tanks and pressurized sections are particularly sensitive areas on aircraft. Punctures cannot be tolerated in either area, but fasteners penetrating the skin and protruding into a fuel tank area have been shown to constitute a fuel ignition source even without a skin puncture. Unprotected fasteners are a preferred attachment point for the lightning and spark because the energy cannot be distributed fast enough into the surrounding graphite epoxy material (due to its low thermal and electrical conductivity).
Representative of the use of lightning protection sytems in the prior art literature are U.S. Pat. Nos. 3,755,713; 4,479,163; and 4,502,092, assigned to The Boeing Company. Knitted wire mesh is utilized in the lightning protection system of U.S. Pat. No. 3,755,713, in contrast to the preferred embodiments of U.S. Pat. Nos. 4,479,163 and 4,502,092, which utilize nickel-plated graphite fibers. U.S. Pat. No. 3,755,713 shows only a decorative layer of knitted wire mesh, this layer providing insufficient coverage for any fastener heads. Further, this patent discloses no insulation between the fasteners and the structure since the reference structure is Fiberglas and, therefore, nonconductive.
U.S. Pat. No. 4,479,163 discloses an integral lightning protection system for composite aircraft skins. In this system, the composite graphite epoxy skin is covered by a nickel- plated graphite epoxy fabric comprising individual nickel-plated graphite fibers which are woven into the outer layer of fabric on the structural components requiring protection. Depending upon the protection desired (for Zone 1 or Zone 2), a varying percentage of the fiber tows in both the warp and fill direction of the fabric can be plated (for example, every second tow or every third tow).
The composite skin of the aircraft is secured to internal structural members by fasteners that extend inwardly through holes through the skin. The head of the fastener is preferably flush with the outer surface of the graphite epoxy fabric ply. Thereafter, a coating of primer and paint can be applied to the fiber ply.
In one embodiment of U.S. Pat. No. 4,479,163, a fastener passes through a hole in the skin that is lined with a cylindrical Fiberglas insulator. The graphite epoxy fiber fabric is then overlaid by a dielectric plastic strip, and the plastic strip is covered with a coating of primer and paint.
In U.S. Pat. No. 4,502,092, counterbore fasteners extend inwardly through the skin, where they are attached to structural elements. In various embodiments, the structural elements can be electrically insulated from the fastener, the structural elements themselves can be made from nonconductive material, and/or the internal panels, which are fastened in the interior of the compartment defined by the skin, can be electrically insulated from the structural support members attached to the skin. In each of the various embodiments of this patent, however, the fasteners are countersunk into the skin by forming a hole through the already-existing layer of graphite epoxy fiber fabric, inserting the fastener, and filling the remaining hole with potting compound. A dielectric layer can be attached over the graphite epoxy fiber fabric and potting compound layer, followed by a coating of primer and paint.
For a variety of reasons, such as accidental damage to the skin, it may be necessary to repair a graphite epoxy fiber fabric ply. It has been found in practice that following such a repair, the area of the repair is particularly susceptible to lightning strikes, even if the graphite epoxy structure is covered by at least one layer of metal-plated graphite fiber fabric. The reason is that state-of-the-art adhesives are nonconductive and, therefore, inhibit the dispersion of lightning currents out of the repair area. In this area of concentrated current, when the dielectric adhesive used to make the repair breaks down, internal and external arcing results, presenting a fuel ignition hazard and producing structural damage.